Fix atjson-mode round-trip edge-cases

[DavidBuchanan314]

Current behaviours:

>>> cbrrr.encode_dag_cbor({"$bytes": "aa", "abc": "def"}, atjson_mode=True)
b'\xa2cabccdeff$bytesbaa' # should raise exception, not valid under atproto data model
>>> cbrrr.decode_dag_cbor(cbrrr.encode_dag_cbor({"$bytes": False}), atjson_mode=True)
{'$bytes': False} # should raise exception during decode, not valid under atproto data model

(same for $link)

[DavidBuchanan314]

I like having the ability to work with "weird" objects, for testing/edge-case-poking purposes. I might introduce a new mode, atjson_mode="strict", which forbids these things. It can also forbid floats.

Or maybe atjson_mode="sloppy" can retain the looser behaviour, with atjson_mode=True being maximally strict.

[DavidBuchanan314]

Concretely, strict decode should:

• Reject $link, $bytes as object keys
• Reject floats

Strict encode should:

• Reject objects with $link or $bytes keys that also have other keys
• Reject floats